Both single mode and multimode optical fibers must be connected to each other and/or to other optical elements to provide an optical signal path. The fibers must be accurately positioned with respect to the other optical elements in order to promote efficient light coupling between optical elements so that optical devices will be practical. Further, the intersecting, relative positions and the alignment of the fibers must be durably maintained in order to provide reliable devices as the devices can encounter a broad range of environmental stresses and fluctuations. In some applications the exact position of the fiber's endface must be maintained with respect to each of the six possible degrees of motion which are X, Y and Z positions in three-dimensional space and motions of roll, pitch and yaw. Some applications require not only that the position of the fiber's endface be maintained, but also that its longitudinal, peripheral surface extending from the endface be maintained in a constant position with respect to other surfaces.
Optical fibers have been attached to other optical elements in a variety of ways well known in the art. These include the use of ultraviolet cured adhesives, laser fusing, soldering, and mechanical structures. Microetched, grooved surfaces have been used in conjunction with UV cured adhesives to fix the fiber in place. Metal coated and uncoated fibers have been aligned while surrounded in molten solder. The solder is cooled and solidified after the desired alignment is obtained. This approach has also been used for UV cured adhesives. A soldering technique is illustrated in U.S. Pat. No. 4,033,668.
Any single mode attachment method is difficult because the positional tolerances are so small, on the order of tenth's of microns, and this is particularly so when it is necessary to accomplish the repetitive alignment and bonding of optical fibers in side by side relationship to form an array.
Organic adhesives are difficult to dispense with reproducable uniformity, quantity and distribution. Organic materials degrade at relatively low temperatures and outgassing of organic materials can affect the performance of other components. Some organic materials are slow to set or cure.
Laser fusing can be adapted to the bonding of one fiber at a time but generates severe thermal gradients that can alter fiber alignment and even damage other optical elements. Thermal fusion of the fiber to a glass support is less adaptable to the bonding of one fiber at a time and also creates thermal disturbances which can alter fiber alignment. The soldering of uncoated and metal coated fibers may be satisfactory for a single fiber, but is difficult to apply to an array of fibers that must be fixed precisely one fiber at a time. Use of structures with mechanical grooves to guide the alignment of the fibers prior to a broad area bond is possible but has limited application and is highly susceptible to manufacturing and tolerance errors.